An LRA is a common-place motor used in a variety of applications (including haptics or force-feedback applications). Generally, an LRA has a mass that is secured to a spring, and the mass is moved by use of a coil that is located in proximity to the mass. As a result of their construction, LRAs have a resonant frequency, and, at this resonant frequency, the LRA can be driven efficiently. However, the efficiency falls off sharply as the drive frequency moves away from the resonant frequency of the LRA. For example (as shown in FIG. 1), the vibrational strength is decreased by 25% if the drive frequency is ±2.5 Hz from the resonant frequency (i.e., 175 Hz). Moreover, the resonant frequency of an LRA is not constant; there can be a frequency shift that can be caused by a number of environmental factors (such as mechanical wear, temperature, and LRA orientation or position). As a result this frequency shift, driving an LRA at a substantially constant drive frequency would result in poor efficiency.
One conventional method that has been employed in an attempt to combat some of the issues associated with driving an LRA can be seen in FIG. 2. For this method, a drive interval with a predetermined or pre-defined length is employed. Typically, the LRA is driven over this drive interval. Following the drive interval the driver is shut-off or placed in a high impedance state to allow the back electromotive force (back-emf) to be monitored during a “monitor interval.” Once the back-emf reaches a predetermined threshold (in what can be referred to as a “zero-crossing”), a measurement of the back-emf is made after a delay interval, which is subtracted from the input signal. The LRA is then driven over the drive interval having the predetermined length.
A problem with the method is that, when the drive period is divided into its four quadrants T1 to T4 (as shown in FIG. 3), the LRA is only driven during the quadrants T1 and T3. This means that the LRA has a drive period that is less than one-quarter of its resonant period during quadrants T1 and T3 and has about one-quarter of its resonant period during quadrants T2 and T4. The total drive period is, thus, less than the resonant period, resulting in a drive frequency that is greater than the resonant frequency.
Another problem with this method relates to braking Typically, a gain is applied to the back-emf value (measured after the delay interval) and subtracted from the input signal to obtain the output drive amplitude. If the gain is large, the drive amplitude tends to be smaller for the same input, so it is undesirable to have a large gain. However, if the gain is small, braking is weaker and more ineffective because, at the time of braking, the input signal is zero and the output amplitude is negative.
Therefore, there is a need for a method and/or apparatus for driving an LRA with improved performance.
Some examples of conventional systems are: U.S. Pat. No. 7,843,277; and U.S. Patent Pre-Grant Publ. No. 2010/0153845.